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 <h2 class="title">require_once</h2>
 <p class="verinfo">(PHP 4, PHP 5, PHP 7, PHP 8)</p>
 <p class="para">
  <code class="literal">require_once</code> 表达式和 <span class="function"><a href="function.require.html" class="function">require</a></span>
  表达式完全相同，唯一区别是 PHP 会检查该文件是否已经被包含过，如果是则不会再次包含。
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 <p class="para">
  参见 <span class="function"><a href="function.include-once.html" class="function">include_once</a></span> 的文档来理解
  <code class="literal">_once</code> 的含义，并理解与没有 <code class="literal">_once</code> 时候有什么不同。
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